Florence Piola

Niche construction by the invasive Asian knotweeds (species complex Fallopia): impact on activity, abundance and community structure of denitrifiers and nitrifiers.

Big Asian knotweeds (Fallopia spp.) are among the most invasive plant species in north-western Europe. We suggest that their success is partially explained by biological and chemical niche construction. In this paper, we explored the microbial mechanisms by which the plant modifies the nitrogen cycle. We found that Fallopiaspp. decreased potential denitrification enzyme activity (DEA) by reducing soil moisture and reducing denitrifying bacteria density in the soil. The plant also reduced potential ammonia and nitrite oxydizing bacteria enzyme activities (respectively, AOEA and NOEA) in sites with high AOEA and NOEA in uninvaded situation. Modification of AOEA and NOEA were not correlated to modifications of the density of implicated bacteria. AOB and Nitrobacter-like NOB community genetic structures were significantly different in respectively two and three of the four tested sites while the genetic structure of denitrifying bacteria was not affected by invasion in none of the tested sites. Modification of nitrification and denitrification functioning in invaded soils could lead to reduced nitrogen loss from the ecosystem through nitrate leaching or volatilization of nitrous oxides and dinitrogen and could be considered as a niche construction mechanism of Fallopia.

Scent evolution in Chinese roses

The phenolic methyl ether 3,5-dimethoxytoluene (DMT) is a major scent compound of many modern rose varieties, and its fragrance participates in the characteristic “tea scent” that gave their name to Tea and Hybrid Tea roses. Among wild roses, phenolic methyl ether (PME) biosynthesis is restricted to Chinese rose species, but the progenitors of modern roses included both European and Chinese species (e.g., Rosa chinensis cv Old Blush), so this trait was transmitted to their hybrid progeny. The last steps of the biosynthetic pathways leading to DMT involve two methylation reactions catalyzed by the highly similar orcinol O-methyltransferases (OOMT) 1 and 2. OOMT1 and OOMT2 enzymes exhibit different substrate specificities that are consistent with their operating sequentially in DMT biosynthesis. Here, we show that these different substrate specificities are mostly due to a single amino acid polymorphism in the phenolic substrate binding site of OOMTs. An analysis of the OOMT gene family in 18 species representing the diversity of the genus Rosa indicated that only Chinese roses possess both the OOMT2 and the OOMT1 genes. In addition, we provide evidence that the Chinese-rose-specific OOMT1 genes most probably evolved from an OOMT2-like gene that has homologues in the genomes of all extant roses. We propose that the emergence of the OOMT1 gene may have been a critical step in the evolution of scent production in Chinese roses.

Evidence for biological denitrification inhibition (BDI) by plant secondary metabolites.

Previous studies on the effect of secondary metabolites on the functioning of rhizosphere microbial communities have often focused on aspects of the nitrogen (N) cycle but have overlooked biological denitrification inhibition (BDI), which can affect plant N-nutrition. Here, we investigated the BDI by the compounds of Fallopia spp., an invasive weed shown to be associated with a low potential denitrification of the soil. Fallopia spp. extracts were characterized by chromatographic analysis and were used to test the BDI effects on the metabolic and respiratory activities of denitrifying bacteria, under aerobic and anaerobic (denitrification) conditions. The BDI of Fallopia spp. extracts was tested on a complex soil community by measuring denitrification enzyme activity (DEA), substrate induced respiration (SIR), as well as abundances of denitrifiers and total bacteria. In 15 strains of denitrifying bacteria, extracts led to a greater BDI (92%) than respiration inhibition (50%). Anaerobic metabolic activity reduction was correlated with catechin concentrations and the BDI was dose dependent. In soil, extracts reduced the DEA/SIR ratio without affecting the denitrifiers: total bacteria ratio. We show that secondary metabolite(s) from Fallopia spp. inhibit denitrification. This provides new insight into plant-soil interactions and improves our understanding of a plants ability to shape microbial soil functioning.

Rapid detection of genetic variation within and among in vitro propagated cedar (Cedrus libani Loudon) clones

Random amplified polymorphic DNA (RAPD) was examined as a tool to estimate genetic variation within and among in vitro propagated Cedrus libani clones. Eleven arbitrary 10-mer primers were successfully used to amplify DNA of microcuttings obtained from four mother-plantlets. No genetic variation was detected within microcutting clones and DNA profiling with RAPDs allowed discrimination between the four intraspecific mother-plantlets.

The domestication process of the Modern Rose: genetic structure and allelic composition of the rose complex

Abstract Genetic variability among 100 old cultivated rose varieties from 13 horticultural groups was estimated by arbitrary primed (AP) PCR. Using five long (20-mer) PCR primers, 58 polymorphic DNA fragments were produced, of which 55 were highly discriminant, allowing differentiation of the quasi-totality of the 100 cultivars. A dendrogram was constructed displaying the relative genetic similarities between cultivars estimated from the presence/absence of PCR fragments. It shows the relationships between the Chinese and European founder roses, hybrid groups of the first (Bourbons, Noisettes, Portlands) and second (Hybrid Perpetuals and Teas) generations, and the most modern Hybrid Teas, produced during the history of domestication. Principal components analysis (PCA) of the same data demonstrates the occurrence of a continuous gradient of the European/Chinese allele ratio, and a considerable reduction of genetic variability superimposed with the progress of domestication. The two complementary analyses are in good agreement with the horticultural literature. They also give access to DNA fragments potentially linked to genes involved in the control of the main morphogenetic characters of various groups.

Achene buoyancy and germinability of the terrestrial invasive Fallopia × bohemica in aquatic environment: a new vector of dispersion?

Abstract: Seed dispersal along watercourses can favour the long-distance migration of invasive species, not only for aquatic or wetland species, but also for terrestrial wind-dispersed plants. It is crucial to investigate the role of watercourses in the dispersal of the knotweed hybrid complex (Fallopia × bohemica) due to its frequent occurrence on riverbanks and production of fertile achenes. For this purpose, we experimentally studied buoyancy and germinability of F. × bohemica achenes in stirred water, over 28 d. We also measured the long-term survival and growth of seedlings according to exposure time to water. After nearly 2 d in the water, 50% of achenes were still afloat. After 3 d, germination occurred in water and the seedlings also floated. Moreover, the exposure of achenes to water, for however long, significantly favoured their germination rate, without affecting seedling survival, compared to a direct planting in soil. Furthermore, a maximum seedling dry mass was reached following exposure to water for 277 h (11.5 d), surpassing significantly the dry mass of seedlings planted directly in soil. Water exposure strongly favours achene germination and seedling survival. Our results demonstrate a high potential for the seeds and seedlings of Fallopia to be dispersed successfully by water.

Inference of reticulation in outcrossing allopolyploid taxa: caveats, likelihood and perspectives

Abstract Single- or low-copy nuclear sequences are now widely employed for phylogenetic reconstruction. In a new paper, Ferguson and Sang apply this approach to allotetraploid species of peonies Paeonia and document the first case of natural homoploid hybrid speciation between tetraploid taxa. This surprising finding could shed light on the relationship between the two main pathways of hybrid speciation: allopolyploidization and homoploid hybrid speciation. In addition, this work illustrates both the promise and uncertainty associated with the reconstruction of reticulate phylogenies using molecular tools.

An allelochemical from Myrica gale with strong phytotoxic activity against highly invasive Fallopia x bohemica taxa

We report the identification of the allelochemical 3-(1-oxo-3-phenylpropyl)-1,1,5-trimethylcyclo-hexane-2,4,6-trione, known as myrigalone A, from the fruits and leaves of Myrica gale. The structure of the compound was confirmed by high-resolution techniques (UV, MS and NMR analysis). The compound is phytotoxic towards classical plant species used for allelochemical assays and also against Fallopia x bohemica, a highly invasive plant. Application of either powdered dry leaves or dry fruits of M. gale also showed in vitro phytotoxic activity. We hypothesize that M. gale could be used as a green allelopathic shield to control Fallopia x bohemica invasion, in addition to its potential use as an environmentally friendly herbicide.

Effects of endogenous ABA levels and temperature on cedar (Cedrus libani Loudon) bud dormancy in vitro

Abstract Axillary and apical buds of in-vitro-propagated cuttings of Cedrus libani are unable to burst at 24 °C, but this inhibition was overcome at 30 °C. Here we have used cedar microcuttings to investigate whether the levels of endogenous hormones vary with bud dormancy and temperature. We analysed the levels of abscisic acid, indole-3-acetic acid, zeatin, isopentenyladenine and their major metabolites using HPLC purification and fractionation of the samples coupled to an ELISA method for hormonal quantitation involving several antibodies elicited against each hormonal family. Abscisic acid levels in microcuttings with dormant buds were higher than those in microcuttings with growing buds. At 24 °C, needles accumulated more abscisic acid than at 30 °C. In addition, when needles were removed, but growth release was achieved at 24 °C. Abscisic acid supplied at 30 °C induced the formation of dormant buds. These results suggest that abscisic acid accumulation in the needles can explain the bud dormancy of cedar microcuttings at 24 °C.

Identification of B-type procyanidins in Fallopia spp. involved in biological denitrification inhibition.

Nitrogen (N) is considered as a main limiting factor in plant growth, and nitrogen losses through denitrification can be responsible for severe decreases in plant productivity. Recently, it was demonstrated that Fallopia spp. is responsible for biological denitrification inhibition (BDI) through the release of unknown secondary metabolites. Here, we investigate the secondary metabolites involved in the BDI of Fallopia spp. The antioxidant, protein precipitation capability of Fallopia spp. extracts was measured in relation to the aerobic respiration and denitrification of two bacteria (Gram positive and Gram negative). Proanthocyanidin concentrations were estimated. Proanthocyanidins in extracts were characterized by chromatographic analysis, purified and tested on the bacterial denitrification and aerobic respiration of two bacterial strains. The effect of commercial procyanidins on denitrification was tested on two different soil types. Denitrification and aerobic respiration inhibition were correlated with protein precipitation capacity and concentration of proanthocyanidins but not to antioxidant capacity. These proanthocyanidins were B-type procyanidins that inhibited denitrification more than the aerobic respiration of bacteria. In addition, procyanidins also inhibited soil microbial denitrification. We demonstrate that procyanidins are involved in the BDI of Fallopia spp. Our results pave the way to a better understanding of plant-microbe interactions and highlight future applications for a more sustainable agriculture.